Metal-air electrochemical power sources, particularly Al-air batteries and fuel cells with alkaline electrolyte are suitable for electric vehicles, unmanned aerial vehicles (UAV), reserve and emergency power supply and other applications.
Metal-air system with alkaline electrolyte has a great electrochemical capacity (about 8 k Wh/kg). However, during the operation of metal-Air batteries, metal hydroxide such as aluminum hydroxide and soluble ions such as aluminates (e.g. K+[Al(OH)4]−) in the case of aluminum-air battery are formed in the electrolyte solution by dissolution of metal from the anode. This process lowers the efficiency of the metal-air battery. Accordingly, following a certain operation time, the electrolyte solution needs to be replaced or regenerated.
One factor that makes electrolyte regeneration challenging is that most of the aluminum in the spent electrolyte is present in the form of aluminates and therefore is difficult to separate from the used electrolyte solution.
In its most general form, the operation of metal/air electrochemical cell is based on the reduction of oxygen, which takes place at the cathode, and on the oxidation of metallic anode. The aqueous electrolyte present in the cell is preferably a highly alkaline solution, e.g., highly concentrated potassium hydroxide solution. A typical structure of a metal/air battery is schematically shown in FIG. 1, in which the air cathode, the consumable metallic anode and the electrolyte are shown. These components (the cathode, anode and electrolyte) are described in more detail below.
A commonly used air cathode consists of (i) an electronically conductive screen, an expanded foil or a metallic foam which serves as a current collector, (ii) active electrode particles provided within or surrounding the current collector (including a catalyst for promoting the reduction of oxygen) and (iii) hydrophobic porous film (for example PTFE, Teflon®) supported on one face of said screen or foil. One face of the air cathode is exposed to oxygen source (e.g., air) and the other face of the air cathode is exposed to the alkaline electrolyte. The air cathode is permeable to air, but its external face is hydrophobic and impermeable to the aqueous electrolyte.
The anode immersed in the electrolyte is made of metals such as aluminum, zinc, magnesium, iron and alloys thereof. When aluminum anode is used, then the cell is a primary cell, i.e., recharging of the cell is effected by replacing the spent aluminum anode with a fresh anode. In the case of zinc anode, both primary and secondary cells are known.
Turning now to the electrolyte, in aluminum/air batteries for example, it is generally held in a reservoir placed externally to the battery, and it flows to and from the cell stack utilizing a suitable circulation system. It is noted that the oxidation reaction of an aluminum anode in an alkaline electrolyte (e.g., potassium hydroxide) results in the formation of the aluminate ion [Al(OH)4]− as shown below:4Al(s)+3O2(g)+6H2O+4KOH(aq)→4K+(aq)+4Al(OH)4(aq)−  (I)
During discharge, i.e., energy generation, as the concentration of the aluminate within the recirculating electrolyte increases, the battery voltage decreases, due to the reduction in the ionic conductivity of the electrolyte and lack of free hydroxide ions. Thus, the operability of the electrolyte solution deteriorates gradually with time of operation and once it drops below an acceptable level, the spent electrolyte consisting of the aluminate solution is removed from the reservoir and fresh alkaline electrolyte is introduced into the reservoir.
In U.S. Pat. No. 4,908,281 it is explained that after the dissolved aluminate exceeds saturation level, the precipitation of solids takes place in the recirculating alkaline electrolyte due to the following reaction:4K+(aq)+4Al(OH)4(aq)−→4Al(OH)3(solid)+4KOH(aq)  (II)
Reaction (II) is therefore supposed to release potassium hydroxide from the corresponding aluminate and concurrently form a precipitate of aluminum hydroxide. However, experimental work carried out at our laboratories in connection with the present invention indicates that the spent aluminate-containing electrolyte is not easily separable into potassium hydroxide and aluminum hydroxide. FIG. 2 is a bar diagram illustrating the composition of a fresh electrolyte consisting of 30% w/w aqueous potassium hydroxide solution (left bar) and a spent electrolyte withdrawn from an aluminum/air battery (right bar). The results indicate that in the spent electrolyte most of the potassium hydroxide is bound within the potassium aluminate, with only minor fraction being available in a free form (KOH free). Likewise, the quantity of the solid phase (the aluminate-containing precipitate) is small.
Consequently, the release of potassium hydroxide from spent electrolyte, such that it may be recycled and reused in the metal/air battery, poses a challenge to the rapidly developing electric vehicle industry where such batteries are employed for powering vehicles. A feasible method for regenerating potassium hydroxide from spent potassium aluminate solution would constitute a major advancement in metal/air battery technology.